Refactor/group type

[collinc97]

Changes:

Implements: #26, #30

Syntax:

function main() {
  // Definitions
  let a = 0group;
  let b = (0, 1)group;

  // Expressions
  let c = a + b;
  let d = a - b;
  let e: bool = a == b;
  let f = if e ? a : b;
  
  // Statements
  assert_eq!(a, b);
}

Function inputs supported function main(g: group)
Function returns supported function main() -> group

Functionality:

New GroupType trait defines an affine point on the curve passed into the Leo compiler. The set of affine points on the curve defines a group.

pub trait GroupType<F: Field>:
    Sized
    + Clone
    + Debug
    + EqGadget<F>
    + ConditionalEqGadget<F>
    + AllocGadget<String, F>
    + CondSelectGadget<F>
    + ToBitsGadget<F>
    + ToBytesGadget<F>
{
    fn constant(string: String) -> Result<Self, GroupError>;

    fn add<CS: ConstraintSystem<F>>(&self, cs: CS, other: &Self) -> Result<Self, GroupError>;

    fn sub<CS: ConstraintSystem<F>>(&self, cs: CS, other: &Self) -> Result<Self, GroupError>;
}

An example implementation of GroupType can be found at /compiler/group/edwards_bls12.rs